1. Field of the Invention
The present invention relates to a fuel cell provided with a fuel electrode and an air electrode, which sandwich an electrolyte, and to a method for operating a fuel cell. Specifically, the present invention relates to a direct methanol fuel cell using methanol as fuel for power generation, and to a method for operating a fuel cell.
2. Description of Related Art
A fuel cell is a power generating device for generating electric power by electrochemically reacting fuel with oxygen (oxidant gas). The fuel cell has attracted attention in recent years as a power generating device that does not pollute the environment, because the product of power generation performed therein is mainly water. There are attempts to use fuel cells as, for example, a drive power source for driving an automobile, and a household cogeneration system.
Fuel cells for use as drive power sources in, for instance, portable electronic devices, such as a notebook personal computer, a portable phone, and a PDA (Personal Digital Assistant), are energetically developed, in addition to those for drive power sources for driving automobiles. It is important for such fuel cells to stably output necessary electric power and to have a portable size and weight. Various techniques are energetically developed so as to meet such requirements.
Fuel cells are classified into various types according to kinds of electrolyte, fuel supply methods, and so on. A direct methanol fuel cell (DMFC) using methanol as fuel without being reformed into hydrogen has been proposed. In the DMFC, at an anode side, mainly the following reaction occurs:CH3OH+H2O→CO2+6H++6e−.At a cathode side, mainly the following reaction occurs:3/2O2+6H++6e−→3H2O.
Protons (H+) generated at the anode side are transported through an electrolyte to the cathode side. Thus, the following overall reaction occurs to generate water and carbon dioxide together with electricity:CH3OH+3/2O2→CO2+2H2O.
The DMFC requires supplying a fuel, which is a mixture of water and methanol, to the anode, because an electricity generating reaction does not proceed at the anode even if pure methanol is simply supplied as fuel thereto. As a fuel supply method to be performed at that time, a method of preliminarily mixing methanol and water at an appropriate composition ratio has been proposed. Further, another method has been proposed, wherein a fuel channel, in which fuel is circulated, is provided and pure methanol is replenished thereinto thereby to compensate for shortage of methanol component, which is caused by consumption thereof in the electricity generating reaction, and wherein water generated in the electricity generating reaction is collected and mixed into the fuel.